The present invention relates to electromagnetic chucks in general and more particularly to an electromagnetic chuck power supply and controller.
Electromagnetic chucks are used on various machine tools such as, for example, milling machines, drill presses, lathes and surface grinders for holding a workpiece in position while a machining operation is effected upon the workpiece. Electromagnetic chucks comprise one or a plurality of electrical coils inducing magnetic flux lines in electromagnet cores made of a ferromagnetic material such as low carbon, high permeable steel or cast iron. The electrical coils are wound in such direction and the holding surfaces of the diverse electromagnets are arranged such that areas of opposite magnetic polarities are engaged by the workpiece, with the result that the workpiece, also made of ferromagnetic material, is held on the surface of the electromagnet cores.
When it is desired to release the workpiece, the chuck coils are de-energized, such as to permit the removal of the workpiece from engagement with the chuck surface. However, as the workpiece is made of ferromagnetic material, generally steel, it becomes magnetized to a certain degree and the residual magnetism of the workpiece may be such as to make it difficult to remove the workpiece from the chuck surface, unless the workpiece is, at least partially, de-magnetized.
As the coils of electromagnetic chucks are energized with direct current, a source of direct current is thus required. The direct current is generally obtained from an AC/DC converter, through rectification to direct current of the alternating current generally obtainable from the mains. Conversion of alternating current to direct current is achieved by way of vacuum tubes, diode rectifier bridges, and the like. De-magnetization of the workpiece is achieved by reversing the direction of flow of the direct current in the chuck coils, such as to knock down the residual magnetism in the workpiece. De-magnetization of the workpiece may be accomplished manually by operating a reversing switch, and applying direct current, generally at a reduced voltage, across the chuck coils for a short period of time, of the order of a few seconds or less, for example. However, with large workpieces, the holding force from residual magnetism may be so great as to prevent removal of the workpiece with a single one-step de-magnetization pulse. In order to accomplish full de-magnetization of the workpiece, a series of alternating pulses of progressively decreasing amplitude is required.
Adjustably variable holding force is a desirable convenient feature in an electromagnetic chuck. It permits the use to controllably adjust the DC voltage or current to the chuck such that the magnetic flux and, consequently, the holding force may be controllably adjusted. Such a feature is convenient if it is desired to maintain a holding force sufficient to hold a workpiece on the chuck during machining of the workpiece, but not so strong as to cause shape deformation of the workpiece. In conventional electromagnetic chucks, variable holding power is accomplished for example by using variable or multiple dropping resistors, or multi-tapped transformers, or by using phase angle fired silicon rectifiers. However, the use of resistors or multi-tap transformers requires mechanical devices such as rotary switches manually operated or operated by stepping motors. The use of phase angle fired SCR to vary the power applied to the electromagnetic chuck is subject to fairly erratic performance from surges or variations in the AC line voltage. Failure of an SCR eliminates power to the chuck, with the resulting safety hazard of complete loss of workpiece holding force, or reduction of the holding force to that caused by residual magnetization.
The present invention provides a controllable power supply for electromagnetic chucks which, through the use of a magnetic amplifier controlled by a DC current flowing in the control winding of a saturable reactor magnetic amplifier, permits to adjustably vary the power supplied to the load circuit, the electromagnetic chuck coils, from zero to 100% by varying the intensity of the DC control current flowing through the control winding of the magnetic amplifier. In addition, in the event of failure of the DC control circuit, full output power is applied to the electromagnetic chuck coils, as the load power delivered at the output of the magnetic amplifier is inversely proportional to the control current intensity. Furthermore, the present invention provides automatic de-magnetization of the workpiece held by an electromagnetic chuck in applications where such a feature is desirable to facilitate removal of the workpiece from the chuck.